A lithium ion battery has been used as a power source of portable electronic devices and the like. Lithium-ion batteries originally had many problems such as inadequate battery capacity and a short charge-discharge cycle life. At present, such problems have been overcome and the application of lithium ion batteries have extended from light electrical appliances such as mobile phones, notebook computers and digital cameras to heavy electrical appliances such as power tools and electric vehicles. In addition, lithium ion batteries are expected to be used particularly as a power source for automobiles, and research and development on an electrode material, a cell structure and the like have been actively conducted.
The lithium ion batteries used as a power source for automobiles are required to be excellent in charge-discharge cycle characteristics at a low temperature, storage characteristics at a high temperature, and cycle characteristics at a high temperature, and to have low resistance and high coulomb efficiency, and various approaches have been taken to address each of the challenges.
For example, Japanese Patent No. 5270050 (US 2014/057166 A1; Patent Document 1) discloses composite graphite particles having a carbonaceous layer on the surface of graphite, which are useful for a negative electrode material and which can provide a lithium ion battery having good cycle characteristics during low-current charge and discharge, input-output characteristics, and high-current cycle characteristics; a method for manufacturing the same; and an electrode sheet and a lithium ion battery using the same.
A current graphite-based negative electrode material has a problem that it decomposes PC serving as a solvent of the electrolyte at the time of charging, resulting in a significant decrease in the initial charge-discharge efficiency in some cases. It is expected that an electrolyte using PC undergoes further growth, and there is an urgent need to develop a negative electrode material which has less impact on PC.
Japanese Patent No. 4781659 (Patent Document 2) has developed a negative electrode material that deactivates the graphite surface to thereby improve the coulomb efficiency, and reduces impact on PC, by subjecting a carbonaceous material containing vanadium to heat treatment at 3,000° C. or higher. Use of PC in an electrolyte makes it possible to provide lithium ion batteries being more excellent in battery characteristics at a low temperature.
In JP 2002-241118 (Patent Document 3), the carbon surface is coated with vanadium oxide, but has had no effect other than improving the initial efficiency.